It all depends on your insurance company, but on the whole you're probably going to be better off telling them if they ask.
That said, my insurance agent told me that I should just hang onto any and all receipts, if anything were to happen then present them to the company and they should be able to work with you.
A good friend of mine had a 91 MR2 we did a bunch of work to, V6 swap, suspension, brakes, wheels, exhaust, you name it. He got t-boned by a buick station wagon and the car was a total write-off. His insurance company initially wanted to give him the book value of the car, around $4k. He made up a list of all the modifications to the car, and met with the company. They ended up paying him for the full value of the car & mods, and selling him the wreck for the salvage value.